Joe Lundberg

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Pacific Takes Control of Main Weather Features Starting This Weekend

October 11, 2012; 9:15 AM ET

Thursday, 11:15 a.m.

At our daily long-range discussions, we'll occasionally look way out into the future to get an idea of seasonal trends, not just the usual daily and weekly trends. It is well established that one of the factors that influences those seasonal forecasts is the state of water temperature profiles in the Pacific Ocean. The last two winter seasons we've dealt with La Nina. Since late winter, however, the water temperatures off the coast of South America have warmed considerably. Over the past three months, most of the central and eastern equatorial Pacific has seen water temperatures consistently above average, though not by a lot. This would translate to an El Nino if it were to grow a little more and be sustained into the start of winter.

Of late, though, those warm anomalies have begun to fade, leading one to question if there will be an El Nino this year. Look at the trend of the sea surface temperature anomalies for the various Nino regions:

For all practical purposes, you might call it a non-signal, or, at best, a very, very weak one.

If La Nina and El Nino conditions are generally absent, what, then, will be the dominant signal going into the winter? The answer may be variable. What we think will happen is that from time to time, one strong signal will emerge to sort of 'dictate' what happens for a while. Then it will likely fade, and another will step up to the plate and try to take control of the pattern. And this could well be the story of the entire winter. Some things will be pretty much etched in stone, like the fact we remain in a cold PDO pattern.

Normally, when the atmosphere goes into a blocking mode in the North Atlantic, you'd forecast a fair amount of cold air in the pattern. Here's the latest NAO chart:

It's been persistently negative since the middle of September, though not terribly so over the past couple of weeks. Still, we have seen a few shots of chilly air invade the U.S. during that time frame, and given the look of the ensemble forecasts going through the next couple of weeks, you'd have to believe there's more cold air in the pipeline.

That said, once we get past the start of the weekend and the coldest air mass so far this season for the Northeast, we're going to see the cold retreat. I won't go back over the reasons why today, as there were outlined in yesterday's post. Suffice it to say that the storm bringing some wet weather to southern California today and tonight, then part of the central and southern Rockies tomorrow and tomorrow night, will draw a growing area of warm, humid air out of central and South Texas and spread it out across the Mississippi Valley and, eventually, into the East Sunday into Monday.

That's the water vapor satellite image of the Northern Hemisphere! Focus in, if you will, on three things. One is the swirl just off the southern California coast. That's the storm I talked about in the paragraph above. The second object is south of the central Aleutians, a similar storm that is at least a 978mb low in looking at the latest ship and buoy reports in the area. Lastly, notice the streamer of moisture that stretches from the southeast side of that storm to the west-southwest. That feature is essentially tracing out the main jet stream flow across that part of the globe.

I take the time to point that feature out because I believe it will strengthen in the coming days and send a series of disturbance to and through the Northwest that will have an impact on the weather downstream across the U.S. later next week.

Another 'signal' we'll look at is something call the East Pacific Oscillation, or EPO. Suffice it to say that when it is positive, we see a strong jet streaking across the central and eastern Pacific at the West Coast of the United States, and it often will translate into stormy weather, and a lot of mild air that gets sent over the Rockies and out onto the Plains. Here's the forecast of the EPO going forward:

I've been seeing this feature on the longer-range computer models for days now, but now it's entering the shorter-term forecast range, and it would seem it is going to exert a bigger influence on the pattern than what's going on downstream in the Atlantic.

For starters, the development of this feature and its migration, if you will, toward the West Coast is the impetus to kick the upper-level low off the California coast out of the way and shove it downstream with increasing speed. I've already laid out how it will grab a growing area of warmth and moisture and send it steadily downstream across the Plains and Mississippi Valley, then farther east later in the weekend.

Behind it, there's no connection to the arctic any longer, so it will remain mild on the Plains to start next week. All the while this is happening, the Northwest will finally get wet thanks to a series of disturbances that will be embedded in this fast jet stream flow. The first will send rain into western Washington and parts of Washington Oregon tomorrow into tomorrow night. It'll be quickly followed by another system later Saturday, aimed more at northwestern Washington and Vancouver Island.

Still a stronger push of moisture appears likely on Sunday, again aimed more at Washington rather than Oregon, but very wet, nonetheless. Because of the fairly northerly track of these features, the underbelly of them will be mild, and that mild air will flood the West and Rockies. Look at the latest GFS ensemble temperature forecasts for Monday:

There may be some downstream blocking in the Atlantic, but if the jet stream coming in from the Pacific is that strong and is carrying that much moisture and warmth with it, it's going to win. That's what appears to be the setup for much of next week.

However, if that EPO forecast is to be believed, and the farther out in time you go, the harder it is to buy it lock, stock and barrel, then that strong Pacific jet will begin to break down late next week. Between now and then colder air will be building in Alaska, and as the jet stream buckles, you would have to think it will finally open up the door to allow colder air back into the pattern. As we have seen over the past few weeks, the odds would favor much of that to dump into the northern Rockies and northern and central Plains, with less of it reaching the East Coast over time.

The views expressed are those of the author and not necessarily those of AccuWeather, Inc. or AccuWeather.com

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Joe Lundberg
Joe Lundberg, a veteran AccuWeather.com forecaster and meteorologist, covers both short and long-term U.S. weather on this blog.